The present invention relates to a cell transfer method and a cell receiving apparatus for use in communication or the like of an asynchronous transfer mode.
FIG. 1 is a schematic diagram illustrating a cell receiving apparatus using a conventional cell transfer method and FIG. 2 illustrates a format of a cell used in the conventional cell transfer method. In this description of a prior art, a case where voice data as information is transferred in a stereophonic mode is described by way of example. In FIG. 2, CH represents a cell header, SN a sequence number, and R and L voice data for R and L channels, respectively. Voice data is a sample of 125 .mu.s and one sample is one octet. One cell includes 20 samples for each of R and L channels and can carry data for 2.5 ms. Transmission of a cell is made every 2.5 ms. In FIG. 1, numeral 1 denotes a cell disassembling circuit which disassembles an input cell into a sequence number, R-channel data, and L-channel data. Numerals 2 and 3 denote an R-channel buffer and an L-channel buffer, respectively, which store data for corresponding channels together with the sequence number. Numerals 4 and 5 denote reading control circuits for R and L channels, respectively. They supply reading signals to the buffers 2 and 3 when data of the number indicated by a sequence number counter 6 are stored in the buffers 2 and 3. Numeral 7 denotes a sequence number counter control circuit which increments the sequence number counter every 2.5 ms during which voice data for one cell is produced.
Operation of the above prior art is now described. In FIG. 1, when a cell is inputted, the cell disassembling circuit 1 disassembles the inputted cell into the sequence number, the R-channel data and the L-channel data. Data for the respective channels are stored in the R-channel buffer 2 and the L-channel buffer 3 together with the sequence number. On the other hand, on the reading side of the buffers 2 and 3, the R-channel reading control circuit 4 or the L-channel reading control circuit 5 produces a reading signal to data having the sequence number indicated by the sequence number counter 6, so that an R data is outputted from the R-channel buffer 2 or an L data is outputted from the L-channel buffer 3. Since the sequence number counter 6 is incremented every 2.5 ms by the sequence number counter control circuit 7, data for each channel is outputted successively. The reading control circuits 4 and 5 operate to read only data having the number indicated by the sequence number counter 6 and accordingly even when a cell having the sequence number of 3 is missing in a network as shown in FIG. 3, for example, any data is not produced when the counter 6 indicates the number 3 and hence data of a cell having a different sequence number is not reproduced in error.
As described above, even in the conventional cell transfer method, voices for a plurality of channels can be synchronized to be transferred and reproduced.
In the conventional cell transfer method, however, since data for all channels are contained in one cell, there is a problem that it is limited to cope with multiple channels. More particularly, if it is assumed that an area for voice data of the cell is 40 octets and one voice sample is expressed by one octet, for example, only 40 channels can be transferred at maximum.
Further, there is a problem that the format of the cell must be changed in accordance with the number of channels to be handled. For example, the cell format for two-channel voice can not cope with three channels or more as it is. Moreover, the utilization rate of cell is reduced to about a half if voice for only one channel is to be transferred.